Conventional centrifugal impelling devices, such as pumps and compressors, utilize a set of rotating vanes, constituting an impeller, operating in a stationary casing. The rotating vanes accelerate the incoming fluid to a higher velocity. The fluid is discharged from the periphery of the impeller and the major portion of the velocity energy is then converted into pressure energy by means of the stationary casing. However, in certain specific applications, such as jet pump drives for watercraft, it is desirable to produce relatively low differential pressures and retain most of the velocity energy imparted to the fluid so that it may be expelled from the casing as a high velocity jet.
Centrifugal impellers are generally classified, according to the major direction of flow in reference to the axis of rotation, as radial flow, axial flow, or mixed flow. Mixed flow impellers combine radial and axial flow characteristics and are widely applied where relatively high fluid flow rates must be delivered at relatively low differential pressures. Such applications include, for example, the aforementioned jet pump drives for watercraft.
The phenomenon known as cavitation is of critical importance in impellers employed in liquid service and may be described as follows. As the liquid entering the impeller is accelerated by the impeller vanes, the pressure of the liquid drops suddenly due to the increase in velocity. Should the absolute pressure of the liquid at the impeller inlet drop below the vapor pressure of the liquid at the operating temperature, some of the liquid will vaporize and bubbles of vapor will be carried into the impeller. These vapor bubbles will collapse violently at some point downstream of the impeller inlet, usually within the impeller itself. The collapse of these vapor bubbles in the impeller produces excessive noise and vibration, and often physically damages impeller surfaces. In addition to the physical damage it causes, cavitation diminishes impeller performance and results in undesirable discharge pressure fluctuations as vapor bubbles form and subsequently collapse.
Attempts have been made to minimize cavitation by eliminating the sudden acceleration of the liquid as it enters the impeller. Screw type devices of gradually increasing diameter in the direction of flow, known generally as inducers, have been installed on impeller inlets. These devices gradually accelerate the liquid and induce it to rotate as it approaches the impeller vanes. The rapid shock acceleration associated with cavitation is thus reduced or eliminated.
Inducers are well suited for use on radial flow impellers which generally have defined inlet ports providing both space for the installation of an inducer and adequate clearance between the leading edge of the vanes and the face of the impeller. In contrast to radial flow impellers, mixed flow impellers generally lack defined inlet ports since the leading edges of the vanes often project forward from the impeller hub into the fluid flow stream. Consequently, inducers are not well suited for installation on mixed flow impellers.
Additionally, despite being somewhat successful in minimizing cavitation, inducers are a device which must be attached to conventional impellers and represent added manufacturing costs and an additional component which may require maintenance or replacement over time.
Therefore, there is a need for a mixed flow impelling apparatus which provides satisfactory performance while minimizing the likelihood of cavitation in liquid service.